1. Field of the Invention
The invention relates to a method and monitoring device for monitoring fluid in subsea equipment, where the monitoring device comprises a sensing element which is in contact with the fluid.
2. Description of the Related Art
Quality monitoring of liquids in remote apparatus and constructions, in particular in subsea equipment, is a challenging task. Such equipment can comprise transformers, power grids, switchgears and the like. By monitoring the properties of such liquids, for example their acidity, or water content a forecast can be made on the aging process of the liquids and the entire apparatus or construction.
It may, for example, be desirable to monitor the quality of a dielectric, i.e., electrically insulating, fluid in subsea equipment that contains electrical elements. Such a dielectric fluid can in particular be a transformer oil that is usually a highly refined mineral oil or a synthetic ester that is stable at high temperatures and has excellent electrical insulating properties. Such dielectric fluids are, for example, used in oil-filled transformers, high voltage capacitors, fluorescent lamp ballasts, high voltage switches and circuit breakers. The functions of the dielectric fluid comprise electrical insulation, suppression of corona and arcing, cooling and for several use cases such as subsea applications to provide pressure compensation.
Since the 1990s, on-line monitoring of dielectric fluids in transformers has become increasingly popular to reduce the number of time-consuming diagnostic operations. There are many techniques that have been developed and implemented to meet the demands of operating companies regarding both monitoring and diagnostics methods. The most common methods are presented in the following.
One known method is dissolved gas analysis (DGA). In dissolved gas analysis, the concentrations of H2, CH4, C2H6, C2H4, C2H2, CO and CO2 are measured to detect a degradation of the transformer fluid that might lead to a fault. Within the remote condition, monitoring DGA based systems implement either gas chromatography, for example, utilizing the SITRAM© gas chromatograph by Siemens, or photo-acoustic spectroscopy, for example, utilizing a Kelman monitoring device by General Electric. Both techniques require dissolved gases separation from the transformer oil. This is not feasible under high pressure.
Another method is partial discharge (PD) monitoring utilizing glass fiber rods, electrical methods, for example, utilizing RF coils or phase impulse current, or acoustic methods. Such PD activity monitoring is a convenient tool to detect the transformer fluid insulation degradation. Nevertheless, it does not provide any information on the composition of dielectric fluid contaminants. Also, PD monitoring is not utilized in a high pressure environment such as the subsea environment.
There are also methods for detecting the cellulose and oil moisture content, for example, via dielectric response analysis, capacitive probes, fiber optical methods and the Karl Fischer titration.
Other methods focus on the degree of polymerization (DP) by the utilization of paper samples and furanic compounds analysis. Also, there are acidity tests and dielectric strength tests. Karl Fischer titration, DP measurements with paper samples, acidity tests and dielectric strength tests are not feasible for in-situ implementation. Dielectric response analysis requires transformer shutdown. Further a capacitive sensor with the specification MMT162 for determining moisture in oil is available from Vaisala, with a metal version withstanding a pressure up to 200 bar. Even such a sensor is not sufficient for all subsea environments and furthermore it just allows for determination of moisture content.
There are also methods utilizing spectroscopy or transparency measurements in the ultraviolet-visible (UV VIS) or the near infrared (NIR) or the mid infrared spectral (MIR) spectral range.
For example, the MIR spectroscopy is a standardized technique for the inspection of insulating oil in the laboratory. However, this inspection is not intended for the determination of the various constituents of an oil (see ASTM D 2144). The technique is more developed for lubricants (see ASTM E 2412).
Further, document CN 201859115 describes optical absorption measurements utilizing a waveguide designed for multichannel fluid spectrum analyzation. However, this analyzer is not adapted for subsea applications.
Therefore, up-to-date commercially available hardware is not designed for monitoring a fluid, in particular a dielectric fluid, in subsea equipment, and therefore maintenance issues related to the condition of such a fluid remain unresolved.